1. Technical Field
The present disclosure relates to a method for making dimethyl ether by reactive-distillation and, more particularly, to a method for making dimethyl ether by a reactive-distillation tower, a combination of a reactor with a distillation tower.
2. Related Prior Art
The boiling point of dimethyl ether (“DME”, CH3OCH3) is −25° C. at the atmospheric pressure. At the normal temperature, dimethyl ether can be pressurized at 5.4 bars and liquidized for convenience in storage, transportation and use. Dimethyl ether is often used for spray. As shown by statistics, about 500 million tons of dimethyl ether was made around the world in the year of 2009. The production of dimethyl ether is of considerable commercial value. Because the natures of dimethyl ether are like that of liquidated petroleum gas, dimethyl ether can be added into liquidated petroleum gas for replacement. Hence, dimethyl ether is deemed a new energy carrier in the 21st century. Furthermore, the combustion of dimethyl ether is cleaner than that of diesel. Hence, dimethyl ether is more environmentally friendly than fossil fuel.
Dimethyl ether is commercially made by dehydrating methanol all over the world. In a commercial process, there are two devices used, i.e., a reactor and a dimethyl ether separation and purification device. The dimethyl ether separation and purification device is a distillation tower or packing tower in most cases. Methanol is dehydrated in the reactor to provide dimethyl ether and water. The dimethyl ether is collected from a stream produced after the reaction and purified in the dimethyl ether separation and purification device. Because the dehydration is limited by chemical equilibrium, the conversion ratio of the methanol to the dimethyl ether is 80% to 90%. That means, about 20% to 10% of the methanol is wasted, or another separation and purification device must be used to recycle and purify the methanol.
Moreover, as the process for making dimethyl ether by dehydrating methanol is an exothermal reaction, it is conventionally difficult to reuse the heat released from the dehydration. This is a waste of energy.
Reactive distillation has been devised to overcome the foregoing problems. The reactive distillation is based on chemical equilibrium and Le Chatelier's principle. For chemical reactions that are limited by chemical equilibrium, reactants are instantly separated from products. A product with a higher relative volatility is separated from a reactant with a lower relative volatility in a distillation tower. The product with the higher relative volatility is removed from the distillation tower to facilitate further production of the product and reduce risks of side reactions. Therefore, reactive distillation is commercially beneficial.
Reactive distillation processes can be found in US 2007/0066855 A1, US 2009/0048468 A1 and US 2009/0069607 A1 for example. The pressure in a reactive distillation tower must be higher than 8 bars to make dimethyl ether by dehydrating methanol as disclosed in these documents. However, the higher the pressure is, the higher the cost is in constructing and operating the equipment and in consuming energy.
Moreover, in Stanislao et al., 2007, the pressure of the reactive distillation tower must be 8 to 12 bars to make dimethyl ether by dehydrating methanol. The cost is also high in constructing and operating the equipment and in consuming energy.
The present disclosure is therefore intended to obviate or at least alleviate the problems encountered in prior art.